The purpose of this investigation is to analyze how the primate brain develops, so as to provide a basis for understanding and treating the many diseases of the nervous system that arise before birth. Work on the embryonal and fetal brain directly in primates is essential for this purpose because some structures are poorly developed or do not even exist in lower mammals and therefore cannot be studied effectively on the usual laboratory animals. Our effort is mostly concentrated on analysis of the development of highly complex structures of the forebrain which has reached the peak of development in man. The work on the human brain alone, however, provides only a basis for the posing of relevant questions, many of which can be answered only by application of experimental methods in monkeys. In addition the protracted span of development makes primates particularly suitable for unraveling temporal sequences in neurogenesis. The fetal brains are processed by several specialized histological methods, including autoradiography, Golgi staining, and electron microscopy. Particular attention is given to the interactions of brain cells as they multiply and change positions relative to one another at early stages of development. These events appear to determine the fundamental patterns of cell interconnections, the "wiring" arrangements of the brain. The immediate projects under study concern 1) a neuron-glia relationship during migration of granule cell neurons from the external granular layer to the internal granular layer of the cerebellum 2) a guidance mechanism for cells migrating from the subventricular zone to the superficial cortical layers of the parietal lobe 3) kinetics of proliferation and latency between final cell division and onset of differentiation and 4) the histogenesis of occipital, frontal, and parietal cortex 5) development of cortico-ponto- cerebellar system 6) histogenesis of substantia gellationosa of the fetal spinal cord.